U.S. patent application number 10/317631 was filed with the patent office on 2003-07-03 for data processing apparatus and method.
Invention is credited to Pelly, Jason Charles, Tapson, Daniel Warren.
Application Number | 20030123658 10/317631 |
Document ID | / |
Family ID | 9927558 |
Filed Date | 2003-07-03 |
United States Patent
Application |
20030123658 |
Kind Code |
A1 |
Pelly, Jason Charles ; et
al. |
July 3, 2003 |
Data processing apparatus and method
Abstract
A watermarking system comprises an encoding data processor
operable to generate at least one marked version of an original
item of material by introducing one of a predetermined set of code
words into a copy of the original material item. The encoding data
processor is arranged to form other code words of the set by
cyclically shifting a first code word. The system includes a
detecting data processor operable to identify the code word in the
marked material item. The detecting data processor is operable to
recover the code word from the marked material item and to form a
Fourier transform of the recovered code word and a Fourier
transform of the first code word of the set. The data detecting
processor forms correlation samples by forming an inverse transform
of a combination of the recovered and the first code word. Each of
the correlation value samples provides the correlation value for
one of the set of code words. A computationally efficient way of
calculating the correlation for each code word in the set is
thereby provided. As a result a time taken to detect a code word
present in a marked material item is improved. The watermarking
system finds particular application in identifying a point of
distribution of pirate copies of video material generated by
capturing the watermarked image, using, for example, a camcorder in
a cinema.
Inventors: |
Pelly, Jason Charles;
(Reading, GB) ; Tapson, Daniel Warren;
(Basingstoke, GB) |
Correspondence
Address: |
William S. Frommer, Esq.
FROMMER LAWRENCE & HAUG LLP
745 Fifth Avenue
New York
NY
10151
US
|
Family ID: |
9927558 |
Appl. No.: |
10/317631 |
Filed: |
December 12, 2002 |
Current U.S.
Class: |
380/46 |
Current CPC
Class: |
G06T 2201/0052 20130101;
H04N 1/32165 20130101; H04N 1/32154 20130101; H04N 2201/327
20130101; G06T 1/005 20130101; H04N 2201/324 20130101; G06T
2201/0063 20130101; H04N 2201/3236 20130101 |
Class at
Publication: |
380/46 |
International
Class: |
H04L 009/00 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 13, 2001 |
GB |
0129840.5 |
Claims
1. An encoding data processing apparatus for generating at least
one marked version of an original item of material by introducing
one of a predetermined set of code words into a copy of said
material item, said apparatus comprising a code word generator
operable to generate a first code word using a pseudo-random number
generator, the pseudo-random number generator being operable to
generate pseudo-random numbers from which code word coefficients of
the first code word are derived, and an encoding processor operable
to permute an order of the coefficients of a code word for
combining with the material item in accordance with a permutation
code, and to combine the code word coefficients with said material
item in accordance with said permuted order, wherein said code word
generator is operable to generate said pseudo-random numbers for
said first code word coefficients from a seed value for
initialising said pseudo-random number generator, the seed value
uniquely defining said first code word and to generate other code
words of the set from a cyclic shift of said first code word.
2. An encoding data processing apparatus as claimed in claim 1,
wherein said code word generator is operable to generate said seed
value from the samples of said material item.
3. An encoding data processing apparatus as claimed in claim 1 or
2, comprising a discrete cosine transform processor operable to
transform said material item into the discrete cosine transform
domain, said material item in said discrete cosine transform domain
being represented as a plurality of discrete cosine transform
coefficients, wherein said encoding processor is operable to
combine said code word with said material item by adding each of
said code word coefficients to a corresponding one of said discrete
cosine transform coefficients, and an inverse discrete cosine
transform processor operable to form said marked copy of said
material item by performing an inverse discrete cosine transform on
said discrete cosine transformed image to which said code word has
been added by said encoding processor.
4. A cinema projector including an encoding data processing
apparatus according to claim 1, wherein said data processing
apparatus is operable to receive at least one of audio signals and
image signals before reproduction, and to introduce a code word
into said at least one of audio signals and image signals before
reproduction.
5. A web server operable to provide material items for downloading
via the Internet, said web server including an encoding data
processing apparatus according to claim 1, wherein said data
processing apparatus is operable to receive material items and to
introduce a code word into said material items before said material
items are downloaded.
6. A detecting data processing apparatus operable to identify at
least one of a plurality of code words present in a marked version
of a material item, said marked version having been formed by
combining a plurality of samples of an original version of said
material with one of a corresponding plurality of code word
coefficients, said plurality of code words being formed from a
first code word having a plurality of pseudo-randomly distributed
coefficients and by forming other code words by cyclically shifting
said first code word, said apparatus comprising a decoding
processor operable to generate a recovered code word from said
marked material item, and a detection processor operable to detect
said at least one code word from the correlation value for the code
word exceeding a predetermined threshold, wherein said correlation
value is formed for a plurality of said code words by forming a
Fourier transform of the recovered code word, forming a Fourier
transform of the first code word of said set, forming the complex
conjugate of one of the Fourier transform of the recovered code
word and the Fourier transform of the regenerated code word,
forming intermediate product samples by multiplying each of said
Fourier transform samples of said recovered code word and the
corresponding Fourier transform samples of said first code word,
forming correlation samples by forming an inverse transform of said
intermediate product samples, each of said correlation value
samples providing the correlation value for one of said set of code
words.
7. A detecting data processing apparatus as claimed in claim 6,
wherein said decoding processor is operable to generate said
recovered code word by subtracting corresponding samples of said
original material version from said samples of said marked material
version, and to generate, for each of said plurality of code words
a correlation sum by correlating the recovered code word with each
of the plurality of code words.
8. A detecting data processing apparatus as claimed in claim 6,
comprising a registering processor operable to associate samples of
said marked version of said material item with corresponding
samples from said original material item to which corresponding
code word coefficients may have been added.
9. A detecting data processing apparatus as claimed in claim 6,
wherein said correlation processor includes a code word generator
operable to generate said seed value from the samples of said
marked material item.
10. A detecting data processing apparatus as claimed in claim 6,
wherein said code word has been introduced into said material item
in the discrete cosine transform domain, said apparatus comprising
a discrete cosine transform processor operable to transform said
marked material item and said original material item into the
discrete cosine transform domain, wherein said recovery processor
is operable to generate said recovered code word by subtracting
corresponding discrete cosine transform coefficients of said
original material version from discrete cosine transform
coefficients of said marked material version.
11. A system for identifying the recipient of a material item, said
system comprising an encoding data processor according to claim 1,
operable to generate said marked material item by introducing a
code word generated from a seed uniquely identifying said
recipient, and a detecting data processor according to claim 6,
operable to detect with a predetermined false positive probability
the recipient by detecting the presence or absence of the code word
in said material.
12. A method of generating at least one marked version of an
original item of material by introducing one of a predetermined set
of code words into a copy of said original material item, said
method comprising generating a first code word using a
pseudo-random number generator, the pseudo-random number generator
generating pseudo-random numbers from which code word coefficients
of the first code word are derived, the pseudo-random numbers for
said first code word coefficients being generated from a seed value
for initialising said pseudo-random number generator, the seed
value uniquely defining said first code word, permuting an order of
the coefficients of a code word for combining with the material
item in accordance with a permutation code, combining the code word
coefficients with said material item in accordance with said
permuted order, and generating other code words of the set from a
cyclic shift of said first code word.
13. A method of identifying at least one of a plurality of code
words present in a marked version of a material item, said marked
version having been formed by combining a plurality of samples of
an original version of said material with one of a corresponding
plurality of code word coefficients, said plurality of code words
being formed from a first code word having a plurality of
pseudo-randomly distributed coefficients and by forming other code
words by cyclically shifting said first code word, said method
comprising generating a recovered code word from said marked
material item, and detecting said at least one code word from the
correlation value for the code word exceeding a predetermined
threshold, wherein said correlation value is formed for a plurality
of said code words by forming a Fourier transform of the recovered
code word, forming a Fourier transform of the first code word of
said set, forming the complex conjugate of one of the Fourier
transform of the recovered code word and the Fourier transform of
the regenerated code word, forming intermediate product samples by
multiplying each of said Fourier transform samples of said
recovered code word and the corresponding Fourier transform samples
of said first code word, forming correlation samples by forming an
inverse transform of said intermediate product samples, each of
said correlation value samples providing the correlation value for
one of said set of code words.
14. A data signal representing a material item to which a code word
has been added by the data processing apparatus according to claim
1.
15. A data carrier for carrying a data signal according to claim
14.
16. A computer program providing computer executable instructions,
which when loaded onto a data processor configures the data
processor to operate as the encoding data processing apparatus
according to claim 1.
17. A computer program providing computer executable instructions,
which when loaded onto a data processor configures the data
processor to operate as the detecting data processor according to
claim 6.
18. A computer program providing computer executable instructions,
which when loaded on to a data processor causes the data processor
to perform the method according to claim 12.
19. A computer program providing computer executable instructions,
which when loaded on to a data processor causes the data processor
to perform the method according to claim 13.
20. A computer program product having a computer readable medium
having recorded thereon information signals representative of the
computer program claimed in claim 16.
21. A computer program product having a computer readable medium
having recorded thereon information signals representative of the
computer program claimed in claim 17.
22. A receiver operable to receive signals representative of
material items, comprising an encoding data processing apparatus
according to claim 1, operable to combine at least one code word
with the received signals, said code word being provided to
identify uniquely said received signals.
23. An encoding data processing apparatus for generating at least
one marked version of an original item of material by introducing
one of a predetermined set of code words into a copy of said
original material item, said apparatus comprising means for
generating a first code word using a pseudo-random number
generator, the pseudo-random number generator generating
pseudo-random numbers from which code word coefficients of the
first code word are derived, the pseudo-random numbers for said
first code word coefficients being generated from a seed value for
initialising said pseudo-random number generator, the seed value
uniquely defining said first code word, means for permuting an
order of the coefficients of the code word for combining with the
material item in accordance with a permutation code, means for
combining the code word coefficients with said material item in
accordance with said permuted order, and means for generating other
code words of the set from a cyclic shift of said first code
word.
24. A detecting data processing apparatus for identifying at least
one of a plurality of code words present in a marked version of a
material item, said marked version having been formed by combining
a plurality of samples of an original version of said material with
one of a corresponding plurality of code word coefficients, said
plurality of code words being formed from a first code word having
a plurality of pseudo-randomly distributed coefficients and by
forming other code words by cyclically shifting said first code
word, said apparatus comprising means for generating a recovered
code word from said marked material item, and means for detecting
said at least one code word from the correlation value for the code
word exceeding a predetermined threshold, wherein said correlation
value is formed for a plurality of said code words by means for
forming a Fourier transform of the recovered code word, means for
forming a Fourier transform of the first code word of said set,
means for forming the complex conjugate of one of the Fourier
transform of the recovered code word and the Fourier transform of
the regenerated code word, means for forming intermediate product
samples by multiplying each of said Fourier transform samples of
said recovered code word and the corresponding Fourier transform
samples of said first code word, means for forming correlation
samples by forming an inverse transform of said intermediate
product samples, each of said correlation value samples providing
the correlation value for one of said set of code words.
Description
FIELD OF INVENTION
[0001] The present invention relates to encoding data processing
apparatus and methods, which are arranged to embed code words into
versions of material items. In some applications the code words are
used to uniquely identify the material items.
[0002] Correspondingly, the present invention also relates to data
processing apparatus and methods operable to detect one or more
code words, which may be present in a material item.
BACKGROUND OF THE INVENTION
[0003] A process in which information is embedded in material for
the purpose of identifying the material is referred to as
watermarking.
[0004] Identification code words are applied to versions of
material items for the purpose of identifying the version of the
material item. Watermarking can provide, therefore, a facility for
identifying a recipient of a particular version of the material. As
such, if the material is copied or used in a way which is
inconsistent with the wishes of the distributor of the material,
the distributor can identify the material version from the
identification code word and take appropriate action.
[0005] In this description, an item of material, which is copied or
used in a way, which is inconsistent with the wishes of the
originator, owner, creator or distributor of the material, will be
referred to for convenience as an offending item of material or
offending material.
[0006] The material could be any of video, audio, audio/video
material, software programs, digital documents or any type of
information bearing material.
[0007] For a watermarking scheme to be successful, it should be as
difficult as possible for the users to collude in order to mask or
remove the identification code words. It should also be as
difficult as possible for users to collude to alter the
identification code word to the effect that one of the other users
is falsely indicated as the perpetrator of an offending item of
material. Such an attempt by users to collude to either mask the
code word or alter the code word to indicate another user is known
as a collusion attack.
[0008] Any watermarking scheme should be arranged to make it
difficult for users receiving copies of the same material to launch
a successful collusion attack. A watermarking scheme should
therefore with high probability identify a marked material item,
which has been the subject of a collusion attack. This is achieved
by identifying a code word recovered from the offending material.
Conversely, there should be a low probability of not detecting a
code word when a code word is present (false negative probability).
In addition the probability of falsely detecting a user as guilty
of taking part in a collusion attack, when this user has not taken
part, should be as low as possible (false positive
probability).
[0009] U.S. Pat. No. 5,664,018 discloses a watermarking process in
which a plurality of copies of material items are marked with a
digital watermark formed from a code word having a predetermined
number of coefficients. The watermarked material item is for
example an image. The apparatus for introducing the watermark
transforms the image into the Discrete Cosine Transform (DCT)
domain. The digital watermark is formed from a set of randomly
distributed coefficients having a normal distribution. In the DCT
domain each code word coefficient is added to a corresponding one
of the DCT coefficients. The watermarked image is formed by
performing an inverse DCT. A related publication entitled
"Resistance of Digital Watermarks to Collusion Attacks", by J.
Kilian, F. T. Leighton et al, published by MIT, Jul. 27, 1998,
provides a detailed mathematical analysis of this watermarking
process to prove its resistance to attack.
SUMMARY OF INVENTION
[0010] According to an aspect of the present invention there is
provided an encoding data processing apparatus for generating at
least one marked version of an original item of material by
introducing one of a predetermined set of code words into a copy of
the material item. The encoding data processing apparatus comprises
a code word generator operable to generate a first code word using
a pseudo-random number generator. The pseudo-random number
generator is operable to generate pseudo-random numbers from which
code word coefficients of the first code word are derived. The
encoding data processor includes an encoding processor operable to
permute an order of the coefficients of a code word for combining
with the material item in accordance with a permutation code. The
encoding processor combines the code word coefficients with the
material item in accordance with the permuted order. The code word
generator is operable to generate the pseudo-random numbers for the
first code word coefficients from a seed value for initialising the
pseudo-random number generator. The seed value uniquely defines the
first code word. The code word generator generates other code words
of the set from a cyclic shift of the first code word.
[0011] Permuting the code word coefficients provides an advantage
of reducing the likelihood of a successful collusion attack, which
may increase by forming the code words from cyclically shifting the
first code word.
[0012] The present invention aims to provide a practical
watermarking system, which utilises code words having coefficients
which are randomly distributed as proposed as in U.S. Pat. No.
5,664,018. In order to implement a practically useful system the
number of uniquely identifiable code words in the set should be as
high as possible. For a consumer distributed product such as a
video or a film for display at a cinema, there should be in the
order of a million or preferably tens of millions of code words in
the set. As such, it will be appreciated that forming a correlation
of each of the regenerated code words in the set of ten million and
the recovered code word represents a considerable computational
task. As such even for high performance computers, such a
correlation would require an impracticably long time or at least an
inconveniently long time. Embodiments of the present invention are
provided with advantages with respect to calculating the
correlation values for the code words in the set. This is provided
by forming at least some of the code words of the set by generating
a first code word and generating other code words by cyclically
shifting the first code word. As such the correlation values for
all code words of the set can be calculated using a Fourier
transform correlator. As will be explained, the Fourier transform
correlator provides the correlation values for the set in one
operation, substantially reducing the computational task.
[0013] According to an aspect of the present invention there is
provided a data processing apparatus comprising a decoding
processor operable to generate a recovered code word from a marked
material item, and a detection processor operable to detect at
least one code word from marked material item. The code word is
detected from correlation values produced by correlating the
recovered code word with each one of a plurality of regenerated
code words. A code word is detected if the corresponding
correlation value exceeds a predetermined threshold. The
correlation value is formed for a plurality of the code words
by
[0014] forming a Fourier transform of the recovered code word,
[0015] forming a Fourier transform of the first code word of said
set,
[0016] forming the complex conjugate of one of the Fourier
transform of the recovered code word and the Fourier transform of
the regenerated code word,
[0017] forming intermediate product samples by multiplying each of
said Fourier transform samples of said recovered code word and the
corresponding Fourier transform samples of said first code
word,
[0018] forming correlation samples by forming an inverse transform
of said intermediate product samples, each of said correlation
value samples providing the correlation value for one of said set
of code words.
[0019] In some embodiments the detecting data processor is arranged
to reverse a permutation of either the re-generated code word
coefficients or the recovered code word coefficients which may have
been applied to the code word in the marked material item order to
determine the correlation values.
[0020] Various further aspects and features of the present
invention are defined in the appended claims.
BRIEF DESCRIPTION OF DRAWINGS
[0021] Embodiments of the present invention will now be described
by way of example only with reference to the accompanying drawings,
where like parts are provided with corresponding reference
numerals, and in which:
[0022] FIG. 1 is a schematic block diagram of an encoding image
processing apparatus;
[0023] FIG. 2 is a schematic block diagram of a detecting image
processing apparatus;
[0024] FIG. 3A is a representation of an original image, FIG. 3B is
a representation of a marked image and FIG. 3C is the marked image
after registration;
[0025] FIG. 4 is a graphical representation of an example
correlation result for each of a set of N code words;
[0026] FIG. 5A is a graphical representation of samples of the
original image I, FIG. 5B is a graphical representation of samples
of the watermarked image W'; FIG. 5C is a graphical representation
of correlation results for the original image and the watermarked
image with respect to discrete sample shifts;
[0027] FIG. 6 is a schematic block diagram of a correlator forming
part of the detecting data processing apparatus shown in FIG.
2;
[0028] FIG. 7 is a flow diagram of a process for forming
watermarked images performed by the encoding image data processor;
and
[0029] FIG. 8 is a flow diagram of a process for identifying a
watermark from a received marked copy of the image performed by the
detecting data processor of FIG. 2;
DESCRIPTION OF PREFERRED EMBODIMENTS
[0030] Watermarking System Overview
[0031] An example embodiment of the present invention will now be
described with reference to protecting video images. The number of
users to which the video images are to be distributed determines
the number of copies. To each copy an identification code word is
added which identifies the copy assigned to one of the users.
[0032] Video images are one example of material, which can be
protected by embedding a digital code word. Other examples of
material, which can be protected by embedding a code word, include
software programs, digital documents, music, audio signals and any
other information-bearing signal.
[0033] An example of an encoding image processing apparatus, which
is arranged to introduce an identification code word into a copy of
an original image, is shown in FIG. 1. An original image I is
received from a source and stored in a frame store 1. This original
image is to be reproduced as a plurality of water marked copies,
each of which is marked with a uniquely identifiable code word. The
original image is passed to a Discrete Cosine Transform (DCT)
processor 2, which divides the image into 8.times.8 pixel blocks
and forms a DCT of each of the 8.times.8 pixel blocks. The DCT
processor 2 therefore forms a DCT transformed image V.
[0034] In the following description the term "samples" will be used
to refer to discrete samples from which an image (or indeed any
other type of material) is comprised. The samples may be luminance
samples of the image, which is otherwise, produce from the image
pixels. Therefore, where appropriate the terms samples and pixels
are inter-changeable.
[0035] The DCT image V is fed to an encoding processor 4. The
encoding processor 4 also receives identification code words from
an identification code word generator 8.
[0036] The code word generator 8 is provided with a plurality of
seeds, each seed being used to generate one of the corresponding
code words. Each of the generated code words may be embedded in a
copy of the original image to form a watermarked image. The code
word generator 8 is provided with a pseudo random number generator.
The pseudo random number generator produces the code word
coefficients to form a particular code word. In preferred
embodiments the coefficients of the code words are generated in
accordance with a normal distribution. However, the coefficients of
the code word are otherwise predetermined in accordance with the
seed, which is used to initialise the random number generator. Thus
for each code word there is a corresponding seed which is store in
a data store 12. Therefore it will be understood that to generate
the code word X.sup.i, seed.sub.i is retrieved from memory 12 and
used to initialise the random number generator within the code word
generator 8.
[0037] In the following description the DCT version of the original
image is represented as V, where;
V{v.sub.i}={v.sub.1,v.sub.2,v.sub.3,v.sub.4, . . . v.sub.N}
[0038] and v.sub.i are the DCT coefficients of the image. In other
embodiments the samples of the image v.sub.i could represent
samples of the image in the spatial domain or in an alternative
domain.
[0039] Each of the code words X.sup.i comprises a plurality of n
code word coefficients, where;
X.sup.i={x.sub.j.sup.i}={x.sub.1.sup.i,x.sub.2.sup.i,x.sub.3.sup.i,x.sub.4-
.sup.i, . . . x.sub.n.sup.i}
[0040] The number of code word coefficients n corresponds to the
number or samples of the original image V. However, a different
number of coefficients is possible, and will be set in dependence
upon a particular application.
[0041] A vector of code word coefficients X.sup.i forming the i-th
code word is then passed via channel 14 to the encoder 4. The
encoder 4 is arranged to form a watermarked image W.sup.i by adding
the code word X.sup.i to the image V. Effectively, therefore, as
represented in the equation below, each of the code word
coefficients is added to a different one of the coefficients of the
image to form the watermark image W.sup.i.
W.sup.i=V+X.sup.i
W.sup.i=v.sub.1+x.sub.1.sup.i,v.sub.2+x.sub.2.sup.i,v.sub.3+x.sub.3.sup.i,-
v.sub.4+x.sub.4.sup.i, . . . ,v.sub.n+x.sub.n.sup.i
[0042] As shown in FIG. 1, the watermarked images W.sup.i are
formed at the output of the image processing apparatus by an
forming inverse DCT of the image produced at the output of the
encoding processor 4 by the inverse DCT processor 18.
[0043] Therefore as represented in FIG. 1 at the output of the
encoder 4 a set of the watermarked images can be produced. For a
data word of up to 20-bits, one of 10 000 000 code words can be
selected to generate 10 million watermarked W.sup.i versions of the
original image I.
[0044] Although the code word provides the facility for uniquely
identifying a marked copy W.sup.i of the image I, in other
embodiments the 20 bits can provide a facility for communicating
data within the image. As will be appreciated therefore, the 20
bits used to select the identification code word can provide a 20
bit pay-load for communicating data within the image V.
[0045] The encoding image processing apparatus which is arranged to
produce the watermarked images shown in FIG. 1 may be incorporated
into a variety of products for different scenarios in which
embodiments of the present invention find application. For example,
the encoding image processing apparatus may be connected to a web
site or web server from which the watermarked images may be
downloaded. Before downloading a copy of the image, a unique code
word is introduced into the downloaded image, which can be used to
detect the recipient of the downloaded image at some later point in
time.
[0046] In another application the encoding image processor forms
part of a digital cinema projector in which the identification code
word is added during projection of the image at, for example, a
cinema. Thus, the code word is arranged to identify the projector
and the cinema at which the images are being reproduced.
Accordingly, the identification code word can be identified within
a pirate copy produced from the images projected by the cinema
projector in order to identify the projector and the cinema from
which pirate copies were produced. Correspondingly, a watermarked
image may be reproduced as a photograph or printout in which a
reproduction or copy may be made and distributed. Generally
therefore, the distribution of the watermarked images produced by
the encoding image processing apparatus shown in FIG. 1 is
represented by a distribution cloud 19.
[0047] Detecting Processor
[0048] A detecting image processing apparatus which is arranged to
detect one or more of the code words, which may be present in an
offending marked image is shown in FIG. 2. Generally, the image
processor shown in FIG. 2 operates to identify one or more of the
code words, which may be present in an offending copy of the
image.
[0049] The offending version of the watermarked image W' is
received from a source and stored in a frame store 20. Also stored
in the frame store 24 is the original version of the image I, since
the detection process performed by the image processor requires the
original version of the image. The offending watermarked image W'
and the original version of the image are then fed via respective
connecting channels 26, 28 to a registration processor 30.
[0050] As already explained, the offending version of the image W'
may have been produced by photographing or otherwise reproducing a
part of the watermarked image W.sup.i. As such, in order to improve
the likelihood of detecting the identification code word, the
registration processor 30 is arranged to substantially align the
offending image with the original version of the image present in
the data stores 20 and 24. The purpose of this alignment is to
provide a correspondence between the original image samples I and
the corresponding samples of the watermarked image W.sup.i to which
the code word coefficients have been added.
[0051] The effects of the registration are illustrated in FIG. 3.
In FIG. 3 an example of the original image I is shown with respect
to an offending marked version of the image W'. As illustrated in
FIG. 3, the watermarked image W' is offset with respect to the
original image I and this may be due to the relative aspect view of
the camera from which the offending version of the watermarked
image was produced.
[0052] In order to recover a representation of the code word
coefficients, the correct samples of the original image should be
subtracted from the corresponding samples of the marked offending
image. To this end, the two images are aligned. As shown in FIG. 3,
the registered image W" has a peripheral area PA which includes
parts which were not present in the original image.
[0053] As will be appreciated in other embodiments, the
registration processor 30 may not be used because the offending
image W' may be already substantially aligned to the originally
version of the image I, such as, for example, if the offending
version was downloaded via the Internet. Accordingly, the detecting
image processor is provided with an alternative channel 32, which
communicates the marked image directly to the recovery processor
40.
[0054] The registered image W" is received by a recovery processor
40. The recovery processor 40 also receives a copy of the original
image I via a second channel 44. The registered image W" and the
original image I are transformed by a DCT transform processor 46
into the DCT domain. An estimated code word X' is then formed by
subtracting the samples of the DCT domain marked image V' from the
DCT domain samples of the original image V as expressed by the
following equations: 1 X ' = V ' - V = v 1 ' - v 1 , v 2 ' - v 2 ,
v 3 ' - v 3 , v 4 ' - v 4 , , v n ' - v n , = x 1 ' , x 2 ' , x 3 '
, x 4 ' , x n '
[0055] The output of the recovery processor 40 therefore provides
on a connecting channel 50 an estimate of the coefficients of the
code word which is to be identified. The recovered code word X' is
then fed to a first input of a correlator 52. The correlator 52
also receives on a second input the regenerated code words X.sup.i
produced by the code word generator 54. The code word generator 54
operates in the same way as the code word generator 8 which
produces all possible code words of the set, using the
predetermined seeds which identify uniquely the code words from a
store 58.
[0056] The correlator 52 forms n similarity sim(i) values. In one
embodiment, the similarity value is produced by forming a
correlation in accordance with following equation: 2 sim ( i ) = X
i X ' X i X ' = x 1 i x 1 ' + x 2 i x 2 ' + x 3 i x 3 ' + + x n i x
n ' x 1 i x 1 ' + x 2 i x 2 ' + x 3 i x 3 ' + + x n i x '
[0057] Each of the n similarity values sim(i) is then fed to a
detector 60. The detector 60 then analyses the similarity values
sim(i) produced for each of the n possible code words. As an
example, the similarity values produced by the correlator 52 are
shown in FIG. 4 with respect to a threshold TH for each of the
possible code words. As shown in FIG. 4, two code words are above
the threshold, 2001, 12345. As such, the detecting processor
concludes that the watermarked version associated with code word
2001 and code word 12345 must have colluded in order to form the
offending image. Therefore, in accordance with a false positive
detection probability, determined from the population size, which
in this case is 10 million and the watermarking strength .alpha.,
the height of the threshold TH can be set in order to guarantee the
false detection probability. As in the example in FIG. 4, if the
similarity values produced by the correlator 52 exceed the
threshold then, with this false positive probability, the
recipients of the marked image are considered to have colluded to
form the offending watermarked version of the image W.sup.i.
[0058] The following sections illustrate advantages and features of
the operation of the watermarking system illustrated in FIGS. 1 and
2.
[0059] Registration
[0060] The process of aligning the offending marked version of the
image with the copy of the original image comprises correlating the
samples of the original image with respect to the marked image. The
correlation is performed for different shifts of the respective
samples of the images. This is illustrated in FIG. 5.
[0061] FIG. 5A provides an illustration of discrete samples of the
original image I, whereas FIG. 5B provides an illustration of
discrete samples of the offending watermarked image W'. As
illustrated in the FIGS. 5A and 5B, the sampling rate provides a
temporal difference between samples of dt. A result of shifting
each of the sets of samples from the images and correlating the
discrete samples is illustrated in FIG. 5C.
[0062] As shown in FIG. 5C, for a shift of between 6 and 7 samples,
the correlation peak is highest. The offending watermarked image is
therefore shifted by this amount with respect to the original image
to perform registration.
[0063] Fourier Decoding
[0064] As explained, with reference to FIGS. 1 and 2, the
watermarking system can provide a facility for generating 10
million watermarked versions of an original image. This is effected
using a 20-bit watermark value. However, as explained, in order to
detect the presence of one of the code words in an offending
watermarked image, the detecting image processor must correlate
each of the possible code words in the set of 10 million code words
with respect to a recovered code word from the image. As will be
appreciated, this represents a considerable computational task.
[0065] A correlator embodying the present invention provides a
significant advantage in reducing the computational effort and
therefore the time taken to detect the presence of a code word in
an offending watermarked image. A correlator in accordance with the
embodiment of the present invention is illustrated in FIG. 6. The
correlator shown in FIG. 6 takes advantage of an alternative
technique for calculating the correlation sum shown above. In
accordance with this technique the correlation sum is calculated in
accordance with the following equation:
F.sup.-1[F(X')F(X.sup.(1))*],
[0066] where F(A) is the Fourier transform of A and F.sup.-1(A) is
the inverse Fourier transform of A.
[0067] The corrolator 52 shown in FIG. 6 therefore comprises a
first Fourier transform processor 100, and a second Fourier
transform processor 102. Fourier transform processors 100, 102 may
be implemented using Fast Fourier transform algorithms. The second
Fourier transform processor 102 also forms the complex conjugate of
the Fourier transform of the regenerated code word X.sup.1. The
Fourier transform of the recovered code word X' and the complex
conjugate of the Fourier transform of the regenerated code word
X.sup.i are fed to first and seconds inputs of a multiplier 110.
The multiplier 110 multiplies the respective samples from each of
the Fourier transform processors 100, 102 and feeds the multiplied
samples to an inverse Fourier transform processor 112. At the
output of the correlator an inverse Fourier transform of the
multiplied signals samples is formed.
[0068] As will be appreciated, the implementation of the correlator
52 shown in FIG. 6 provides an advantage in terms of time taken to
compute the correlation for the n sample values of the regenerated
code word X.sup.i and the recovered code word X'. This is because
the Fourier processors 100, 102, 112 can be formed from FFT
integrated circuits such as, for example, are available as ASICS.
Furthermore, the inverse Fourier transform provided at the output
of the corrolator 52 provides n similarity values sim(i)
corresponding to n correlation sums. However, in order to utilise
the properties of the corrolator 52, shown in FIG. 6 the code words
are arranged to be generated by cyclically shifting one code word
generated X.sup.(1) using a particular seed for the random number
generator. This is illustrated below. As shown below, the first
code word X.sup.(1) is represented as values x.sub.1 to x.sub.n
which corresponds to the pseudo randomly produced numbers from the
code word generator 8. However, the second code word X.sup.(2) is
produced by performing a cyclic shift on the first code word
X.sup.(1). Correspondingly, each of the other code words are
produced by correspondingly cyclically shifting further the code
word X.sup.(1) until the n-th code word is a code word shifted by
n-1 positions.
X.sup.(1).fwdarw.(x.sub.1,x.sub.2,x.sub.3,x.sub.4 . . .
,x.sub.n-1,x.sub.n)
X.sup.(2).fwdarw.(x.sub.2,x.sub.3,x.sub.4 . . .
,x.sub.n-1,x.sub.n,x.sub.1- )
X.sup.(3).fwdarw.(x.sub.3,x.sub.4 . . .
,x.sub.n-1,x.sub.n,x.sub.1,x.sub.2- )
[0069] -- -- --
X.sup.(n).fwdarw.(x.sub.n,x.sub.1,x.sub.2,x.sub.3,x.sub.4, . . .
x.sub.n-2,x.sub.n-1)
[0070] By using this set of code words to form part of, or the
whole of, the set of code words produced by the encoding image
processor, the Fourier transform correlator 52 can be used to
generate in one operation all similarity values for all of the n
code words. Therefore, as illustrated above, the corresponding
shift of 1 to n of the original code word provides the n similarity
values sim(i), and as illustrated in FIG. 4, for at least one of
the code words, a large similarity value sim(i) is produced.
Therefore, as will be appreciated the correlator 52 only receives
one regenerated code word corresponding to first code word
X.sup.(1) to form the similarity values for the set of n code words
as illustrated in FIG. 4.
[0071] As will be appreciated from the above explanation, if the
code word contains N samples, then only N unique cyclic shifts are
possible. Therefore, if the required population of code words is p,
which is greater than N, then several base watermarks will be
required. Each base watermark can be cyclically shifted to produce
N unique code words.
[0072] If the watermarked image forms one of a plurality of images
in, for example, a video sequence, then the same code word will be
added to each of the images. As such, once the suspected code word
has been identified using the Fourier transform corrolator
illustrated in FIG. 6, then a subsequent correlation can be formed
using the full correlation sum sim(i) as explained above. However,
because the suspected code word has already been identified, then
the correlation only needs to be performed once for the code word
identified by the Fourier transform correlator shown in FIG. 6.
[0073] As will be appreciated, instead of forming the conjugate of
the Fourier transform of the regenerated first code word X.sup.1,
the conjugate of the Fourier transform of the recovered code word
could be formed. This is expressed by the second alternative of the
Fourier transform correlator shown below:
F.sup.-1[F(X')*F(X.sup.(1))]
[0074] Accordingly the conjugate of one of the Fourier transform of
the recovered code word and the Fourier transform of the
regenerated code word is formed by the Fourier transform processors
100, 102.
[0075] Secret Permutation of Code Words
[0076] One disadvantage of forming a code word from a cyclic shift
of a first code word X.sup.1 is that the security of the watermark
may be compromised. This is because under a collusion attack two
watermarked images are compared. If the same code word has been
added to each image, with only a cyclic shift with respect to two
versions of the same code word, an attacker may be more likely to
identify the differences between the two marked material items and
therefore identify the code word. With knowledge of the code word
an attacker may either remove the watermark or alter the watermark
to falsely implicate another.
[0077] In order to reduce the likelihood of a successful collusion
attack, the order of each of the code word coefficients of each of
the cyclically shifted code words is randomly permuted in
accordance with a secret permutation code .pi.. The permutation of
the code word coefficients remains secret from the recipients of
the marked images. Accordingly the likelihood of a successful
collusion attack is reduced by an increase in the difficulty
presented to a collusion attacker of identifying a correlation
between two marked images.
[0078] At the detecting data processor the secret permutation code
.pi. will be known. In the detecting data processor, either the
code word re-generator or the recovery processor 40 is operable to
reverse the permutation .pi..sup.-1 of either the re-generated code
word coefficients or the recovered code word coefficients in order
to perform the correlation. The operation of the encoding data
processor of FIG. 1 and the detecting data processor of FIG. 2 is
therefore as presented in a flow diagrams in FIGS. 7 and 8
respectively.
[0079] Code Word Generation
[0080] A further advantageous aspect of the embodiment shown in
FIGS. 1 and 2 is provided by generating the seed of the random
number from which the code word is produced from the source image
samples. This is affected by analysing the DCT coefficients of the
image to be watermarked and from these coefficients, generating the
seed to be used to generate the code word. This can be effected,
for example, by using a hashing algorithm known to those skilled in
the art as "secure hashing algorithm 1" (sha-1). This algorithm
forms an ANSI standard (ANSI x9.30-2). This algorithm is referred
to in a book entitled "Handbook of applied cryptography" by A. J.
Menezes. As such the seed from the random number can be generated
and determined in the encoding image processor and the detecting
image processor from the DCT coefficients.
[0081] Other Applications
[0082] In addition to the above-mentioned applications of the
encoding data processing apparatus of the watermarking system to a
cinema projector and to a web server, other applications are
envisaged. For example, a receiver/decoder is envisaged in which
received signals are watermarked by introducing code words upon
receipt of the signals from a communicating device. For example, a
set top box is typically arranged to receive television and video
signals from a "head-end" broadcast or multi-cast device. As will
be appreciated in this application, the encoding data processing
apparatus forms part of the set top box and is arranged to
introduce watermark code words into the video signals as the
signals are received and decoded. In one example embodiment, the
watermark code word is arranged to uniquely identify the set top
box which receives and decodes the video signals.
[0083] In a further embodiment a digital cinema receiver is
arranged to receive a digital cinema film via a satellite. The
receiver is arranged to receive signals representing the digital
cinema film and to decode the signals for reproduction. The
receiver includes an encoding data processing apparatus, which
introduces a watermark code word into the decoded film signals. The
watermark code word is provided, for example, to uniquely identify
the cinema receiving the film signals.
[0084] A further example embodiment may comprise a digital camera
or camcorder or the like which includes a memory and a memory
controller. An encoding data processing apparatus according to an
embodiment of the present invention is arranged to introduce a
watermark code word stored in the memory into video signals
captured by the camera. According to this embodiment, the encoding
data processing apparatus does not include a code word generator
because the code word is pre-stored in the memory. Under the
control of the memory controller the code word stored in the memory
is embedded into the video signals, uniquely or quasi-uniquely
identifying the video signals.
[0085] In a further embodiment, an encoding data processing
apparatus according to an embodiment of the invention is operable
to encode a sequence of watermark code words into different frames
of digital images forming a continuous or moving picture. The code
words may be related to one another and may be used to identify
each of the images separately.
[0086] Various further aspects and features of the present
invention are defined in the appended claims. Various modifications
can be made to the embodiments herein before described without
departing from the scope of the present invention.
* * * * *